A spokesman said on Friday morning: "As a result of a conservative and prudent decision, the Sellafield site is operating normally but with reduced manning levels today.

"This follows the detection of elevated levels of radioactivity at one of the on-site radiation monitors at the north end of the site.

"Essential workers only are being asked to report for work."

"Levels of radioactivity detected are above naturally occurring radiation but well below that which would call for any actions to be taken by the workforce on or off the site.

A later statement released by Sellafield said that "following investigation and analysis, we can now confirm these levels to be naturally occurring background radon".

All plants and storage facilities on site were said to be working normally.

Staff are due back in on Monday, the statement added.

There is no active nuclear reactor at Sellafield but the site stores and reprocesses waste.

One worker said an air sampler on a perimeter fence had detected a problem, which led to all non-essential staff being told to stay away. It is understood nothing has been detected inside the plant.

He said employees had not been given any details of what had happened at Sellafield.

Prof Richard Wakeford, Professor of Epidemiology at the University of Manchester, said the elevated level of radiation would not pose a risk to health.

"From the information currently available, it appears that an elevated level of radioactivity has been detected at the north of the site, but that it is at a low level above normal," he said.

"Such a level would not pose a risk to health that is more than encountered in everyday life, but until the cause of this increase has been identified (for example, what type of radioactive materials are responsible), the Sellafield management have told non-essential staff not to come into work.

"This is a prudent precaution until the cause is known and the situation rectified."

A Department of Energy and Climate Change spokesman confirmed the elevated levels of radioactivity but said they were "well below levels of concern".